It is known that artificial fibers, for example, polyester fibers, polyamide fibers, polyacrylic fibers and cellulose acetate fibers, exhibit a very poor electric conductivity and one highly hydrophobic. Therefore, when an artificial fiber mass is rubbed, static electricity is generated and accumulates on the fibers. Sometimes, the voltage of accumulated static electricity reaches a very high level of several kilovolts. This phenomenon results in various annoyances, that is, items of apparel cling to the body and are attracted to other garments; fine particles of lint and dust are attracted to a fabric; the frequency of required cleaning is increased; and a jolt or shock is experienced upon touching a metallic article after walking across a carpet. Also, the above-mentioned accumulation of static electricity on the fiber mass is dangerous that is, the discharge of static electricity result in a spark capable of igniting flammable mixtures, such as an ether-air mixture, which are commonly found in hospitals, especially in operating rooms.
It is also known that natural fibers, such as animal hairs and silk, exhibit a relatively high hydrophilic property and this property causes the natural fibers to exhibit a relatively high electric conductivity at a relatively high humidity due to a relatively high content of moisture absorbed in the natural fibers. This moisture content can avoid the above-mentioned annoyance and danger which is derived from the accumulation of static electricity. However, when the natural fibers are placed in an atmosphere of low humidity, static electricity will build up on the fibers, so as to cause the above-mentioned annoyance and danger.
The most effective manner for preventing the above-mentioned undesirable phenomena is to utilize fibers having a high electrical conductivity. For this purpose, metallic fibers, fibers plated with a metal, fibers containing therein carbon black as an electrically conductive material U.S. Pat. Nos. 3,803,453, 3,969,559 and 4,045,949) and fibers coated with a polymeric dope containing an electrically conductive material (British Pat. Nos. 1,259,315 and 1,391,262) are known as electrically conductive fibers.
However, conventional electrically conductive fibers are not necessarily satisfactory for practical use. For example, metallic fibers exhibit a very poor recovery from bending and are easily broken when processed or used. The breakage results in a decrease in the conductive effect of the metallic fiber. Also, it is difficult to mix spin, mix weave or mix knit the metallic fibers with organic polymer fibers. Further, the metallic fibers have a color which is sometimes undesirable for textile use.
In case of a fiber plated with a metal, it is required to form a uniform, continuous plated metal layer on the surface of the fiber. In order to satisfy the above-mentioned requirement, the surface of the fiber is required to be smooth.
Therefore, the metal plating operation can be applied only to limited types of fibers having a smooth surface.
In order to prepare a continuous layer of plated metal possessing a uniform thickness, it is necessary to carefully and accurately carry out the plating operation. The requirement causes the cost of the plating operation to be very high. Also, the metal-plated fiber is disadvantageous in that the plated metal layer is easily peeled off from the fiber during processing or use, and has a poor durability in use and a metallic color which is sometimes undesirable for textile use.
The fiber coated with a polymer dope containing an electrically conductive material, for example, carbon black and silver particles, is disadvantageous in that the coating operation is expensive and the coating layer is easily peeled off from the fiber during processing or use.
In order to provide the fiber containing therein carbon black as an electrically conductive material and having a proper conductivity, it is necessary that the carbon black is contained in a large amount of at least 15% based on the weight of the fiber matrix polymer. This large amount of carbon black causes the fiber-producing process to be difficult, complex and expensive. Also, it is impossible to contain the carbon black in the inside of the natural fibers. Furthermore, the carbon black-containing fiber has a gray or black color which is sometimes undesirable for textile use.